Most of highly efficient electronics are basically in a rigid and planar shape and use a single crystal inorganic material such as silicon and gallium-arsenic. On the other hand, when a flexible substrate is used, resistance of the wiring against bending is demanded. Moreover, in the use such as actuator electrodes and skin sensors, an electrically conductive material showing high stretchability is demanded. When a membrane of a stretchable electrically conductive material is used for example, it is possible to develop a device which can closely attach to and adapt to human body which is flexible and curvilinear. Use of the device as such ranges from the measurement of electrophysiological signals to the delivery in advanced therapy as well as the interface between humans and machines. One of the solving methods in the development of stretchable electrically conductive materials is to use an organic electrically conductive material. Although the materials known up to now are flexible, they are not stretchable and they cannot cover curvilinear surfaces. Therefore, they are unreliable in their property and their integration into complicated integrated circuits. Membranes of other materials such as metal nanowire and carbon nanotube are favorable to some extent but they are unreliable and expensive whereby development thereof is difficult.
As to approaches for developing a stretchable flexible wiring, two methods have been mainly reported.
One is a method wherein a corrugated structure is constructed so as to make even fragile materials stretchable (see Non-Patent Document 1). In this method, a thin metal membrane is prepared on a silicone rubber by means of vapor deposition, metal plating, photoresist treatment, etc. Although a thin metal membrane shows stretchability of only a few percents, stretchability can be shown when the thin metal membrane is made in a zigzag shape, a continuous horseshoe shape or a corrugated shape, or when the thin metal membrane is made in a wrinkled shape or the like by forming the thin metal membrane on a previously stretched silicone rubber. However, in any of the above, electric conductivity lowers to an extent of two digits or more when the thin metal membrane is stretched by several tens percents. In addition, since silicone rubber has low surface energy, adhesion between the wiring and the substrate is weak whereby there is a disadvantage that detachment is apt to happen upon stretching. Accordingly, in this method, it is difficult to achieve both high electrical conductivity and high stretchability. Moreover, there is another problem that the manufacturing cost is high.
Another approach is a composite material consisting of an electrically conductive material and an elastomer. Advantages of this material are excellent printing property and stretchability. In a commercially available silver paste used for electrodes and wirings, high amount of silver powder is filled in and compounded with a binder resin of high elasticity whereby flexibility is poor and elasticity is high. Upon stretching, cracks are generated and the electrical conductivity significantly lowers. In view of the above, there have been carried out the investigations in rubber and elastomer as a binder for imparting the flexibility and also the investigations in silver flakes, carbon nanotube, metal nanowire, etc. which have large aspect ratio as a conductive material and high electrical conductivity for lowering the filling rate of a conductive material. There have been reported a combination of silver particles with silicone rubber (see Patent Document 1), a combination of silver particles with polyurethane (see Patent Document 2), a combination of carbon nanotube with ionic liquid and vinylidene fluoride (see Patent Documents 3 and 4), etc. However, it is the current status that, even in those combinations, it is still difficult to achieve both high electrical conductivity and high stretchability. On the other hand, there has been reported a composite material which is printable, highly conductive and stretchable by means of a combination of silver particles in a micron size with polyvinylidene fluoride and carbon nanotube which has been subjected to a surface modification with self-organized silver nanoparticles (see Non-Patent Document 2). However, the surface modification or carbon nanotube with silver nanoparticles is not preferred because its manufacture is troublesome causing an increase in cost.